1. Field of the Invention
The present invention relates to a transition metal compound having a substituted cyclopentadienyl group as a ligand and a fluorenyl group as another ligand, both ligands being bridged by a diarylmethylene group. The present invention relates also to a process for producing the transition metal compound. The present invention further relates to an olefin polymerization catalyst employing the transition metal compound. The present invention further relates to a process for producing an olefin polymer using the catalyst.
2. Description of the Related Art
For olefin polymerization, a metallocene catalyst is known which comprises a cyclopentadienyl derivative of a transition metal such as titanium, zirconium, and hafnium (metal of Group 4 of the Periodic Table) and an aluminoxane as basic constituents, as shown in publications such as J.Boor: "Ziegler-Natta Catalyst and Polymerization", Academic Press, New York (1979); and H.Sinn and W.Kaminsky: Adv. Organomet. Chem., 1899 (1980).
In .alpha.-olefin polymerization, it is known that a substituent in the cyclopentadienyl ring ligand, or the bridge between the two cyclopentadienyl ring ligands of the metallocene compound, affects greatly the stereotacticity and molecular weight of the resulting a-olefin polymer. For example, isotactic polypropylene was obtained by use, as the catalyst, of a racemic complex of a transition metal of Group 4 having an ethylenebis(indenyl) ligand in which two indenyl rings are linked by an ethylene bridge, as shown in J. Am. Chem. Soc., 106, 6355, (1984); Angew. Chem. Int. Ed. Engl., 24, 507, (1985); and J. Am. Chem. Soc., 109, 6544 (1987). Syndiotactic polypropylene was obtained by use, as the catalyst, of a transition metal complex of Group 4 having an isopropylidene(cyclopentadienyl) (9-fluorene) ligand constituted of a cyclopentadiene group and a fluorene group bridged by isopropylidene.
Syndiotactic polypropylene was obtained by use, as the propylene polymerization catalyst, of diphenylmethylene(cyclopentadienyl) (fluorenyl) zirconium dichloride which was derived by replacing the dimethylmethylene group of the above transition metal complex of Group 4 having an isopropylidene-(cyclopentadienyl) (9-fluorene) ligand by a diphenylmethylene group, as shown in JP-A-2-274703.
The stereotacticity of poly-.alpha.-olefins is known to be varied over a wide range from atactic to isotactic by use of a transition metal complex containing an isopropylidene(cyclopentadienyl)(9-fluorene) ligand substituted at the 3-position of the cyclopentadiene ring by an alkyl group such as methyl and t-butyl and by changing the kind and number of the substituents. For example, hemiisotactic polypropylene was obtained by use of isopropylidene(3-methylcyclopentadienyl)(fluorenyl)-zirconium dichloride having a methyl group as the substituent at the 3-position of the cyclopentadienyl ring, as shown in Makromol. Chem., Macromol. Symp., 48-49, 235 (1991); and JP-A-3-193796. Isotactic polypropylene was obtained by use of isopropylidene(3-t-butylcyclopentadienyl)(fluorenyl)zirconium dichloride which has similarly a t-butyl group at the 3-position of the cyclopentadienyl group, as shown in JP-A-6-122718.
On the other hand, the transition metal compound having a cyclopentadienyl group substituted at 3-position, a fluorenyl group, and a diphenylmethylene bridge between the cyclopentadienyl group and the fluorenyl group could not efficiently be synthesized. Generally, isopropylidene-(cyclopentadienyl)(9-fluorene) as a ligand is synthesized by reaction of a metal salt of fluorene with 6,6-dimethylfulrene, and a ligand having a substituted cyclopentadienyl group is synthesized in a similar manner. However, the ligand of the above compound cannot readily be synthesized since the reaction does not proceed rapidly between the metal salt of fluorene and the 6,6-dimethylfulrene derivative substituted at the five-membered ring moiety by an electron-donating hydrocarbon substituent.
In polymerization of an .alpha.-olefin, metallocene catalysts generally do not give high molecular weight poly-.alpha.-olefins. For example, syndiotactic, hemitactic, or isotactic polypropylene is obtained, respectively, by use of a catalyst comprising a transition metal compound having a ligand composed of a cyclopentadiene derivative, fluorene, and a dimethylmethylene bridge therebetween, specifically the aforementioned isopropylidene(cyclopentadienyl)(fluorenyl)-zirconium dichloride, isopropylidene(3-methylcyclo-pentadienyl)(fluorenyl)zirconium dichloride, or isopropylidene(3-t-butylcyclopentadienyl)(fluorenyl)-zirconium dichloride, but the molecular weight of the polymer is low.
In copolymerization of ethylene with an .alpha.-olefin, metallocene catalysts produce copolymers of uniform composition distribution differently from conventional Ziegler-Natta catalysts. Various olefin copolymers ranging from linear low-density polyethylene (LLDPE) to an ethylene-propylene copolymer (EPR) can be produced by use of the metallocene catalyst. However, the metallocene catalyst involve s a problem that the molecular weight of the polymer produced at a higher polymerization temperature is significantly lower. JP-A-5-320246 discloses copolymerization of ethylene with 1-octene at a high temperature by use of a cationic transition metal catalyst prepared from dicyclopentadienyl zirconium dichloride, dimethylanilinium tetraphenylborate, and triisobutylaluminum. However, the produced polymer had a low intrinsic viscosity, and a low molecular weight .
In synthesis of olefin type elastomers typified by EPR, the metallocene catalyst involves problems in molecular weight and productivity. Generally, with the metallocene catalyst, the molecular weight of the produced polymer becomes remarkably lower with increase of the amount of .alpha.-olefin. JP-A-62-121709 and JP-A-121711 disclose synthesis examples of low crystalline ethylene/.alpha.-olefin copolymers, and ethylene/.alpha.-olefin/nonconjugated dienes by use of a catalyst prepared from bis(cyclopentadienyl)zirconium monochloride monohydride and methylaluminoxane, in which the polymerization was conducted at a low temperature in order to obtain the desired high molecular weight, but the yield of the polymer was low and the productivity was disadvantageously low. JP-A-5-43618 discloses a production example of EPR by use of a cationic transition metal catalyst prepared from ethylenebis(indenyl)dimethyl-zirconium, ferrocenium tetrakis(pentafluorophenyl)borate, and triisobutylaluminum, but the problem of the molecular weight is not solved.